The present invention relates to an ultrasound echo receiver of the type which is used in medical diagnostic devices.
Ultrasound is used in medical diagnostic equipment in order to perform non-invasive examinations of organs within a patient3 s body. Typically, an ultrasound pulse is transmitted into the patient3 s body, and an echo, representative of the location of a reflecting surface, is returned to the transducer. In view of the fact that the strength of reflected signals falls off exponentially as the distance between the ultrasound transducer and the reflecting surface increases, it is necessary to amplify the echo which is received from the reflecting surface within the diagnostic equipment.
Heretofore, a time-variable gain control (TGC) was used to regulate the RF gain of the ultrasound echos. TGC was used, because echos which are returned from surfaces farther from a transducer are weaker, and therefore require greater gain, than echos which are returned from surfaces closer to the transducer
Problems with the prior art methods of regulating RF gain relate to the fact that it is difficult to eliminate the TGC control voltage from the resulting echo signal, the fact that there is relatively low response speed of RF gain changes to TGC signal changes, and the fact that it is complicated to control RF gain. Typical ultrasound diagnostic receivers use a cathode ray tube (CRT) to display images corresponding to the cross-section of the organs being scanned. The gray scale on a CRT monitor typically has a range of about 40 dB. That 40 dB range is used to represent the strength of received echos having a signal range which extends over approximately 110 dB.
In view of the problems with controlling RF gain in the receiver circuit, it would be desirable to have a circuit which is capable of controlling the gain of the input video signal to the CRT.